1. Field of the Invention
The invention relates to a method and a device for the production of high purity polycrystalline silicon with a reduced dopant content, as well as to the silicon thus produced.
2. Description of the Related Art
High purity polycrystalline silicon (polysilicon) is used as a starting material for the production of monocrystalline silicon for semiconductors according to the Czochralski (CZ) method or the floating zone (FZ) method and for the production of solar cells for photovoltaic applications. It is generally batch-produced using the Siemens process. In this case a reaction gas containing silicon is thermally decomposed or reduced by hydrogen and highly pure silicon is deposited on thin filament rods of silicon, so-called thin rods. The silicon-containing component of the reaction gas may be a halosilane with the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It preferably comprises chlorosilanes (X=Cl), most particularly preferably trichlorosilane (n=1).
The method is carried out in a deposition reactor. In its most common embodiment, the deposition reactor comprises a metal base plate and a coolable bell jar, which is placed onto the base plate so that a reaction space is formed inside the bell jar. The deposition reactor must be hermetically sealed, since the reaction gases have a corrosive effect and are liable to self-ignite or explode when mixed with air. The base plate is provided with one or more supply openings and one or more discharge openings for the reaction gases, as well as with holders which are used to hold the thin rods in the reaction space. Two neighboring rods are generally connected by a bridge at their free ends, on the opposite side from the held base ends, to form a U-shaped carrier body. The U-shaped carrier bodies are heated to the deposition temperature by direct flow of current, and the reaction gas is supplied.
FIG. 1 shows a device according to the prior art for the deposition of polysilicon. In order to deposit polysilicon, the stop valve (8) for the reaction gas (1) flowing to the reactor (4), i.e. a mixture of hydrogen and one of the known silicon-containing components, as well as the stop valve (7) for the waste gas (7) flowing out of the reactor (4) are opened. The reaction gas (1) flows through the supply opening (2) of the base plate (3) into the closed deposition reactor (4). There, silicon is deposited onto the thin rods (not shown) heated by direct flow of current. The hot waste gas thereby formed in the reactor (4) leaves the reactor through the exhaust opening (5) in the base plate (3), and can subsequently be subjected to reprocessing, for example condensation, or supplied to a gas-scrubber.
During the deposition of silicon, the halogenated silicon compounds, for example trichlorosilane, decompose from the gas phase on the surface of the heated thin rods. The diameter of the thin rods thereby grows. After a desired diameter is reached, the deposition is terminated and the polysilicon rods thereby obtained are cooled to room temperature.
After the rods have been cooled, the bell jar is opened and the rods are removed for further processing with the assistance of extraction aids. The bell jar and base plate of the reactor are subsequently cleaned, and provided with new electrodes and thin rods for the next deposition batch. After the bell jar has been sealed, the method is carried out again as described above in order to deposit the next batch of polysilicon.
From the time at which the reactor is opened in order to extract the deposited batch of polysilicon until the reactor is reclosed in order to deposit the subsequent batch of polysilicon, the base plate and the supply and exhaust openings for the reaction gases and the waste gases are exposed to environmental effects, in particular humidity. If moisture enters, particularly into the supply and discharge lines of the reactors, then halosilane residues, for example unreacted reaction gas, or halosilanes or polysilanes formed by the process lead to the formation of corrosive hydrogen halides such as hydrogen chloride.
The hydrogen halide corrodes reactor components, particularly the supply and discharge lines of the reactor. Substances detrimental to the process, for example the electrically active dopants boron, aluminum, phosphorus, arsenic and antimony, are thereby leached from the components of the reactor. During the subsequent deposition, particularly at the start of deposition, an elevated level of these substances will be introduced into the silicon being deposited. This introduction of dopants undesirably modifies the characteristic properties of polysilicon. For example the electrical resistivity, which is crucially determined in silicon by the level of electrically active dopants, is reduced at the start of deposition, which also leads to an inferior radial profile of the electrical resistivity in the deposited poly rod.
In the case of polysilicon produced on a thin rod by deposition, as described, the electrical resistivity on the surface of the thin rod is therefore lowest immediately after the start of deposition. From there, it increases continuously in the direction toward the rod edge until it reaches a plateau. As schematically represented in FIG. 2, this is caused by a phosphorus content decreasing from the thin rod (15) in the direction toward the edge of the polysilicon rod (16) together with a virtually constant boron content. This effect becomes commensurately more detrimental the later this resistance plateau is reached in the course of the deposition. The average electrical resistivity of such a polysilicon rod turns out to be lower, over the entire deposited diameter, than for polysilicon rods with the same final diameter in which this plateau in the radial resistance profile was reached faster. This is particularly problematic when monocrystalline silicon is intended to be produced from the polycrystalline silicon according to the floating zone (FZ) method, which requires a starting material with particularly low dopant values, i.e. an electrical resistivity which is as high as possible.
In order to quantitatively determine and assess the quality of the radial resistance profile, it is highly suitable to use the gradient of the straight line starting from the minimum resistance ρ0 of the polysilicon deposited on the surface of the thin rod immediately after the start of deposition, at the position r=r(ρ0)=0 (definition: r=0 is the start of the poly rod), until the resistance plateau ρ∞ is reached at r=r(ρ∞):
      m    ρ    =                    Δρ        /        Δ            ⁢                          ⁢      r        =                            ρ          ∞                -                  ρ          0                                      r          ⁡                      (                          ρ              ∞                        )                          -                  r          ⁡                      (                          ρ              0                        )                              
The greater the numerical value of mρ is, the faster the plateau in the radial resistance profile will be reached and therefore the higher the average electrical resistivity of the polycrystalline silicon rod will be over the entire deposited diameter.
Here:
    Mρ=gradient of the electrical resistivity profile until the plateau is reached [Ωcm/mm]    ρ∞=electrical resistivity at the start of the plateau [Ωcm]    ρ0=minimum electrical resistivity of the polysilicon deposited on the surface of the thin rod, immediately after the start of deposition [Ωcm],    r(ρ∞)=radius of the polysilicon rod at the start of the plateau [mm]    r(ρ0)=radius of the polysilicon rod on the surface of the thin rod immediately after the start of deposition [mm], which is defined as r=0.
Polysilicon rods produced according to the prior art with a high electrical resistivity for use as FZ polysilicon have values of between 45 and 70 Ωcm/mm for the gradient of the radial resistance profile.
In order to determine the resistance and the level of dopants, samples are usually taken from the polycrystalline silicon according to the standard SEMI MF 1723-1104 (23.10.2003) and are prepared by float zone (FZ) pulling. The resistance is determined according to standard SEMI MF 397-02 (22.10.2003) and the dopants are determined according to standard SEMI MF 1389-0704 (22.10.2003). Said standards are published by: Semiconductor Equipment and Materials International (SEMI®), San Jose, Calif. (USA).
In a method for depositing polycrystalline silicon on a graphite surface, it is known from GB 1532649 to flush the closed reactor with an inert gas, for example argon, before the start of heating the deposition surface or briefly before the deposition. This inert gas flushing to flush out a closed reactor while the process is taking place is carried out for inerting or to avoid explosive gas mixtures (oxygen removal).